Epitaxial layers of Group III-V compound semiconductors--such an InP (indium phosphide), InGaAs (indium gallium arsenide), and InGaAsP--are commonly grown on single crystal substrates, such as InP, by a process known as liquid phase epitaxy (LPE). This process entails bringing molten solution(s) of the corresponding chemical constituents of the desired epitaxial layer(s) into contact with the heated substrate in a furnace. A controlled cooling program causes material to precipitate from the solution onto the substrate and thus to grow the corresponding epitaxial layer. During growth the furnace atmosphere is typically one of hydrogen, helium, nitrogen, argon, or combinations thereof. Typically, the substrate is located in a recess of a sliding member ("slider") which is pushed into contact with different molten solutions ("melts") contained in successive wells in a meltholder, in order to grow a succession of epitaxial layers of differing chemical composition ("heterostructure"). The slider and meltholder are typically made of graphite, and both slider (with substrate) and meltholder (with melt) are initially heated in the furnace to a common temperature for homogenizing the melt(s) prior to pushing the substrate into contact with any melt.
Of particular interest is the LPE growth of Group III-V compound semiconductors such as InP, InGaAs, and InGaAsP because heterostructures of these materials are important for use as light emitting diodes, lasers, photodetectors, field effect transistors, and other devices. The performance of these devices depends to a large degree upon the quality of the substrate and the epitaxial layers. A major problem associated with LPE growth on InP substrates, for example, is the decomposition of the InP caused by the disproportionate loss of phosphorus by dissociation from the InP substrate above 365 degrees C., which occurs when the surface of the substrate is exposed and heated before contact with any melt. This phosphorus loss creates In-rich pits that propagate through the layer as undesirable inclusions which degrade the performance of the devices. Therefore, minimizing InP substrate decomposition is very important.
In prior art, during heating necessary for homogenizing the melt(s) prior to LPE growth, the InP substrate is subjected to a localized partial pressure of phosphorus, as provided by a variety of techniques, to retard InP substrate decomposition. These techniques include the use of an InP cover piece as a source of phosphorus, to create a local phosphorus overpressure and thus suppress loss of phosphorus from the substrate, in a hydrogen (H.sub.2) growth atmosphere. An alternative approach, reported by K. Pak et al, Japan Journal of Applied Physics, Vol. 18, No. 9, page 1859 (1979), is to introduce Ar into the growth ambient, in order to produce Ar-H.sub.2 gas mixtures in the furnace atmosphere. However, neither of these approaches suppresses the decomposition of the InP substrate as much as desired.